When standing in a two block height, with a trap door over your head, opening the drop door above you will force your character to lay down on the block that it is standing on.
You have to stand directly below the hinge for this bug to be reproduced.
The way to get past the issue is to open the trap door, or walk off the block. Then your character will stand up normally.
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We're tracking this issue as MC-147612, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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this is because you're under the collision box of the opened trapdoor